I working on a cost/benefit analysis for PV panels.
Take a 24V-175W panel. Inverted, does this equate to 120V AC @ 35W? What about losses, how many watts do these panels actually produce at the receptacle?
Running the wires, do the run the wires inside the wall or down the outside of the house?
What is the lifespan of the batteries for storage?

The power ratings for solar panels (in watts) reflect the power output of the panel under some sort of standard test condition (usually specified in the fine print). The wattage is the same regardless of the voltage. A 24V 175 watt panel with an inverter to step it up to 120V AC would provide 175 watts at 120V AC but with some caveats:

1. In general, the standard test conditions will not reflect the conditions you have at your site. You may see more or less peak output and the average output power over the course of the day will almost certainly be much less.
2. Your inverter will not be 100% efficient. The actual efficiency can vary quite a bit depending on load and other factors. 90% is typical for a well designed setup.

Wire requirements are driven by building codes. There are safety issues involved.

Most people use lead acid acid batteries for off grid systems. These last between 3 years and 10 years depending on the battery type and how well they are treated. They die much sooner if they experience unfavorable charge discharge cycling (running them dry is particularly bad). The more expensive types last longer.

hychesee

04-08-2009 10:53 AM

Flyboy pretty much nailed the tech part,
Now for some cost info, since the new tax credit the feds implemented this year we decided to do another impromptu cost for a friend in Wisconsin which is one of only a handful of states that offer rebates and incentives. An important note is that in order get any tax credits or rebates the equipment and installation must be installed and bought from certified sources.

12 - 200watt panels = $12,000 and assumes 12 kWh daily for 5 hours of average sun and is a fair estimate for most of the USA but your site needs to be evaluated first.
Grid tied inverter, charge controller and misc electronics = $3000, or if you want to be off grid then assume this to be the battery cost but there are no tax credits or rebates for pure off grid systems.
Misc mounting and accessories = $2000
Installation = $3000
Total = $20,000
Minus state rebate of 25% leaves $15,000
Minus poco rebate leaves $12,000
Minus Fed credit leaves $8,400
Cost of electricity produced monthly 360kWh = $36 @ 10 cent per kWh
Equals a 19 year payback

$10,000 in a 5% CD will net $41 a month in interest and you still have the ten grand.

Remember most states don't have those rebates. Mine has zero rebates.

EDIT: I just noticed MA only has an insolation of 4 hours so subtract 200 watts from all my math, the new payback would be 24 years but probably less because I think you pay more for power.